The present invention resides in a pay television system, and more specifically in the type of pay television system disclosed in U.S. Pat. No. 3,924,059 (Horowitz). In this patent a pay television system is disclosed wherein, in addition to other features, the video signal information is inverted relative to the synchronizing portions of the signal prior to transmission during randomly selected frames. Whether or not the signal is so inverted is determined by the output of a bistable circuit, referred to as a "polarity flip-flop". If the polarity flip-flop is in a SET state, the video signal portion of the composite television signal is inverted, while if it is in a RESET state, no inversion takes place prior to transmission. In order that this inversion, if present, may be compensated for at the receiver, a code signal or burst is added to the mixer at predetermined time instants during the vertical blanking interval, again prior to transmission. At the receiver the code burst sets set a polarity flip-flop to the state corresponding to the state of the polarity flip-flop at the transmitter, thereby causing reinversion of the video portion of the received signal when required.
In a copending application, U.S. application Ser. No. 552,787, filed Feb. 25, 1975, entitled "Coding System for Pay Television Apparatus", a coding system is disclosed wherein code bursts, that is oscillations of different frequencies, rather than just a single code burst, are applied to the television signal within predetermined horizontal line intervals in the vertical blanking interval. In a preferred embodiment disclosed in said application, six code bursts are utilized. A control panel is furnished which allows assignment of any of these code bursts to either an "A" mode, a "B" mode, a "C" mode or a "D" mode. Any code burst assigned to the (A) mode causes the polarity flip-flop to be reset. Any code burst assigned to the "B" causes the polarity flip-flop to be set, while "D" code burst causes a toggling of the polarity flip-flop, that is the polarity flip-flop fade is changed. Code bursts assigned to the "C" mode do not affect the polarity flip-flop at all but are used simply to confuse possible "pirates", that is people wishing to break the code in order to avoid payment of fees. These code bursts are inserted into the composite television signal during predetermined lines of vertical blanking interval. At the receiver the code bursts are detected and used to control a polarity flip-flop which in turn controls the reinversion of the portions of the television signal which were inverted at the transmitter. In the above-identified Loshin application at the transmitter the assignment of each of the tone bursts (eight different tone frequencies are used) to the "A", "B", "C" or "D" mode is carried out by logic circuits which include externally operable switches. At the decoder the tone bursts are filtered out and the corresponding logic circuits are set up in accordance with a punch card having a program corresponding to the code assignment at the transmitter.
The disadvantages of the above-described Loshin system are that the operation of the whole system depends upon the accuracy of the frequencies of the individual tones. But differences in path length can cause distortions in the received frequency, as can interfering elements such as, for example, airplanes. At the receiver, there is some problem with the selectivity of the tuned circuits, which must be high enough to reject extraneous noise signals and at the same time sufficiently broad to include the above-mentioned possible distortions. Further, the cost of the system is relatively high.
A further disadvantage of the known system is that the subscriber must insert the program card carrying the code assignments.